Gas-phase basicity of methionine.

International audienceProton affinity and protonation entropy of methionine (Met) were determined by the extended kinetic method from ESI-Q-TOF tandem mass spectrometry experiments. The values, PA(Met) = 937.5 +/- 2.9 kJ mol(-1) and Delta(p)S degrees (Met) = - 22 +/- 5 J mol(-1) K(-1), lead to gas-phase basicity GB(Met) = 898.2 +/- 3.2 kJ.mol(-1). Quantum chemical calculations using density functional theory confirm that the proton affinity of Met is indeed in the 940 kJ mol(-1) range and that a significant entropy loss, of at least - 25 J mol(-1) K(-1), occurs upon protonation. This last point is evidenced here for the first time and suggests revision of the tabulated protonation thermochemistry of Met. A comparison with previous experimental data allows us to propose the following evaluated thermochemical values: PA(Met) = 943 +/- 4 kJ mol(-1) and Delta(p)S degrees (Met) = - 35 +/- 15 J mol(-1) K(-1) and GB(Met) = 900 +/- 2 kJ mol(-1)

Gas phase protonation thermochemistry of phenylalanine and tyrosine.

International audienceGas phase basicities of phenylalanine and tyrosine, GB(Phe) = 892.0 +/- 1.3(2.6) kJ.mol(-1) and GB(Tyr) = 894.9 +/- 2.8(5.9) kJ.mol(-1) (uncertainties are standard deviation and, in parentheses, 95% confidence limit), have been experimentally determined by the extended kinetic method using ESI-TQ tandem mass spectrometry. Proton affinities deduced from these experiments, PA(Phe) = 931.3 +/- 1.1(2.3) kJ.mol(-1) and PA(Tyr) = 934.8 +/- 2.5(5.2) kJ.mol(-1), are perfectly reproduced by theoretical calculations performed at the B3LYP/6-311++G(3df,2p)//B3LYP/6-31+G(d,p) level. An entropy loss of approximately -25 J.mol(-1).K(-1) occurs upon protonation of both Phe and Tyr. The origin of this entropy change is attributed (i) to the change in strength of the interaction between the amino group and the aromatic moiety in the neutral and protonated forms and (ii) to the larger entropy of mixing associated with the population of neutral conformers with respect to their protonated counterparts. Previous neglect of the protonation entropy term has led to underestimated tabulated PA values; the evaluated values proposed in the present study are PA(Phe) = 932 +/- kJ.mol(-1) and PA(Tyr) = 935 +/- kJ.mol(-1)

Gas-phase protonation thermochemistry of arginine.

International audienceThe gas-phase basicity (GB), proton affinity (PA), and protonation entropy (DeltapS degrees (M)=S degrees (MH+)-S degrees (M)) of arginine (Arg) have been experimentally determined by the extended kinetic method using an electrospray ionization quadrupole time-of-flight (ESI-Q-TOF) mass spectrometer. This method provides GB(Arg)=1004.3+/-2.2 (4.9) kJ.mol(-1) (indicated errors are standard deviations, and in parentheses, 95% confidence limits are given). Consideration of previous experimental data using a fast atom bombardment ionization tandem sector mass spectrometer slightly modifies these estimates since GB(Arg)=1005.9+/-3.1 (6.6) kJ.mol(-1). Lower limits of the proton affinity, PA(Arg)=1046+/-4 (7) kJ.mol(-1), and of the "protonation entropy", DeltapS degrees (Arg)=S degrees (ArgH+)-S degrees (Arg)=-27+/-7 (15) J.mol(-1).K(-1), are also provided by the experiments. Theoretical calculations conducted at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31+G(d,p) level, including 298 K enthalpy correction, predict a proton affinity value of ca. 1053 kJ.mol-1 after consideration of isodesmic proton-transfer reactions with guanidine as the reference base. Computations including explicit treatment of hindered rotations and mixing of conformers confirm that a noticeable entropy loss does occur upon protonation, which leads to a theoretical DeltapS degrees (Arg) term of ca. -45 J.mol(-1).K(-1). The following evaluated thermochemical parameter values are proposed: GB(Arg)=1005+/-3 kJ.mol(-1); PA(Arg)=1051+/-5 kJ.mol(-1), and DeltapS degrees (Arg)=-45+/-12 J.mol(-1).K(-1)